Communication terminal apparatus, base station apparatus, and radio communication method

ABSTRACT

A retransmission request signal creation section ( 119 ) outputs an ACK signal or NACK signal to a NACK signal counting section ( 120 ) based on the result of error detection by an error detection section ( 118 ), the NACK signal counting section ( 120 ) counts, for each communication mode, the number of NACK signals output (that is, the number of data retransmissions) before an ACK signal is output from the retransmission request signal creation section ( 119 ), and a table rewriting section ( 121 ) compares the number of retransmissions counted by the NACK signal counting section ( 120 ) with a predetermined threshold value for the number of retransmissions, and rewrites the contents of a communication mode table ( 102 ) based on the result of this comparison.

This is a Continuation application of Ser. No. 10/110,942 filed Jul. 17,2002 now U.S. Pat. No. 7,006,798.

TECHNICAL FIELD

The present invention relates to a communication terminal apparatus,base station apparatus, and radio communication method to be used in acellular communication system.

BACKGROUND ART

In a cellular communication system, one base station performs radiocommunication with a plurality of communication terminalssimultaneously, and therefore, as demand has increased in recent years,so has the need for higher transmission efficiency.

One technology that has been proposed for increasing the transmissionefficiency of a downlink from a base station to a communication terminalis HDR (High Data Rate). HDR is a communication method whereby a basestation performs scheduling for allocating communication resources tocommunication terminals by time division, and also sets a transmissionrate for each communication terminal according to the downlink channelquality.

The operations by which a base station and communication terminalsperform radio communication with HDR are described below. First, thebase station transmits a pilot signal to each communication terminal.Each communication terminal measures the downlink channel quality usinga CIR (desired signal to interference ratio) based on the pilot signal,etc., and finds a transmission rate at which communication is possible.Then, based on the transmission rate at which communication is possible,each communication terminal selects a communication mode, which is acombination of packet length, coding method, and modulation method, andtransmits a data rate control (hereinafter referred to as “DRC”) signalindicating the communication mode to the base station.

The type of modulation method that can be used in each system ispredetermined as BPSK, QPSK, 16QAM, 64QAM, and so forth. Also, the typeof coding that can be used in each system is predetermined as ½ turbocode, ⅓ turbo code, ¾ turbo code, and so forth. Further, a plurality oftransmission rates that can be used in each system are predeterminedaccording to a combination of packet length, modulation method, andcoding method. Each communication terminal selects a combination wherebycommunication can be performed most efficiently with the currentdownlink channel quality, and transmits a DRC signal indicating theselected communication mode to the base station. Generally, DRC signalsare represented by numbers from 1 to N, with a higher number indicatinga proportionally better downlink channel quality.

Based on the DRC signal transmitted from each communication terminal,the base station sets a transmission rate for each communicationterminal, and sends a signal to each communication terminal via acontrol channel indicating communication resource allocation to eachcommunication terminal. The base station then transmits data only to therelevant communication terminal in its allocated time. For example, iftime t1 has been allocated to communication terminal A, in time t1 thebase station transmits data only to communication terminal A, and doesnot transmit data to a communication terminal other than communicationterminal A.

In this way, data transmission efficiency has conventionally beenincreased for the overall system by setting a transmission rate for eachcommunication terminal according to channel quality by means of HDR, andperforming communication resource allocation preferentially to acommunication terminal with a high transmission rate at whichcommunication is possible.

However, as downlink channel quality measurement in a communicationterminal is performed based on the pilot-part signal within a receivedsignal, if the length of the pilot-part signal is short compared withthe length of the data-part signal, a difference may arise between themeasured channel quality and the current channel quality due to theeffect of fading, etc., while the data-part signal is being received. Ascommunication mode selection is performed based on the measured channelquality, when such a difference arises there is a problem in that thecommunication mode in which communication can be performed mostefficiently with the current channel quality will not be selected, anddownlink throughput will fall.

Also, if error occurs in the channel quality measurement circuit, adifference will arise between the measured channel quality and theactual channel quality, and the same kind of problem as described abovewill arise.

DISCLOSURE OF INVENTION

It is an object of the present invention to provide a communicationterminal apparatus, base station apparatus, and radio communicationmethod that make it possible to prevent a fall in downlink throughput ina communication system in which communication resources are allocated bytime division to communication terminals based on downlink channelquality measured from a pilot signal.

The present inventors arrived at the present invention by consideringthe relationship between a communication mode and the reception qualityof a data-part signal, and finding that, when a difference arisesbetween the measured channel quality and the current actual channelquality, and communication is not performed using the optimalcommunication mode for the current actual channel quality, the receptionquality of the data-part signal either does not attain the desiredreception quality, or else exceeds the desired reception quality.

Thus, in the present invention, the correspondence between downlinkchannel quality and communication mode is changed when a difference isdetected between the measured channel quality and the current actualchannel quality based on the reception quality of the data-part signal.By this means it is possible to perform communication using the optimalcommunication mode for the current actual channel quality.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram-showing the configuration of a communicationterminal according to Embodiment 1 of the present invention;

FIG. 2 is a drawing showing the contents of the communication mode tableprovided in a communication terminal according to Embodiment 1 of thepresent invention;

FIG. 3 is an operational flowchart for explaining the operation of thetable rewriting section provided in a communication terminal accordingto Embodiment 1 of the present invention;

FIG. 4A is drawing showing an example of a communication mode tablerewrite operation by the table rewriting section of a communicationterminal according to Embodiment 1 of the present invention;

FIG. 4B is drawing showing an example of a communication mode tablerewrite operation by the table rewriting section of a communicationterminal according to Embodiment 1 of the present invention;

FIG. 4C is drawing showing an example of a communication mode tablerewrite operation by the table rewriting section of a communicationterminal according to Embodiment 1 of the present invention;

FIG. 5 is a block diagram showing the configuration of a communicationterminal according to Embodiment 2 of the present invention;

FIG. 6 is a block diagram showing the configuration of a communicationterminal according to Embodiment 3 of the present invention;

FIG. 7 is a block diagram showing the configuration of a communicationterminal that performs radio communication with a base station accordingto Embodiment 4 of the present invention;

FIG. 8 is a block diagram showing the configuration of a base stationaccording to Embodiment 4 of the present invention;

FIG. 9 is a block diagram showing the configuration of a communicationterminal that performs radio communication with a base station accordingto Embodiment 5 of the present invention;

FIG. 10 is a block diagram showing the configuration of a base stationaccording to Embodiment 5 of the present invention;

FIG. 11 is a block diagram showing the configuration of a communicationterminal that performs radio communication with a base station accordingto Embodiment 6 of the present invention;

FIG. 12 is a block diagram showing the configuration of a base stationaccording to Embodiment 6 of the present invention;

FIG. 13 is a block diagram showing the configuration of a base stationaccording to Embodiment 7 of the present invention; and

FIG. 14 is a block diagram showing the configuration of a communicationterminal according to Embodiment 7 of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

With reference now to the accompanying drawings, embodiments of thepresent invention will be explained in detail below.

(Embodiment 1)

In a system in which data communication is performed, error control isnormally performed by means of the ARQ (Automatic Repeat reQuest)method. With the ARQ method, data with check bits for error detection,such as CRC (Cyclic Redundancy Check) bits, added is transmitted from abase station to a communication terminal, and if an error is notdetected in the received data, the communication terminal requests thenext data by sending an ACK (ACKnowledgment) signal back to the basestation. If, on the other hand, an error is detected in the receiveddata, the communication terminal sends a NACK (Negative ACKnowledgment)signal back to the base station, and the base station retransmits thedata in which an error was detected. This kind of retransmission isrepeated by the base station until an ACK signal is received for thedata in which an error was detected.

A communication terminal according to Embodiment 1 of the presentinvention is a communication terminal used in a communication system inwhich error control is performed by means of this kind of ARQ method,and determines the reception quality of a data-part signal according tothe number of returns of a NACK signal, and based on the number ofreturns of a NACK signal, rewrites the contents of a communication modetable that indicates the correspondence between the downlink channelquality and communication mode.

FIG. 1 is a block diagram showing the configuration of a communicationterminal according to Embodiment 1 of the present invention. In FIG. 1,a communication mode selector 101 refers to a communication mode table102 and selects a communication mode based on a CIR measured by a CIRmeasurement section 114 described later herein, and outputs this to aDRC signal creation section 103 and NACK signal counting section 120.Also, based on the selected communication mode, the communication modeselector 101 indicates the outbound receive data demodulation method toan adaptive demodulator 116 and indicates the outbound receive datadecoding method to an adaptive decoding section 117. The contents of thecommunication mode table 102 will be described later herein.

The DRC signal creation section 103 creates a DRC signal with a numbercorresponding to the communication mode output from the communicationmode selector 101, and outputs this signal to a modulator 104.

Modulator 104 modulates the DRC signal and outputs it to a spreader 105.Spreader 105 spreads the output signal from modulator 104, and outputsthe resulting signal to a multiplexer 108.

A modulator 106 modulates an ACK signal or NACK signal created by aretransmission request signal creation section 119 described laterherein, and outputs this signal to a spreader 107. Spreader 107 spreadsthe output signal from modulator 106, and outputs the resulting signalto the multiplexer 108.

The multiplexer 108 multiplexes the spread DRC signal and the spread ACKsignal or NACK signal, and outputs them to a transmit RF section 109.The transmit RF section 109 converts the frequency of the output signalfrom the multiplexer 108 to radio frequency, and outputs the resultingsignal to a duplexer 110.

The duplexer 110 transmits the output signal from the transmit RFsection 109 to the base station as a radio signal via an antenna 111. Inaddition, the duplexer 110 outputs a signal transmitted as a radiosignal from the base station and received as a radio signal by theantenna 111 to a receive RF section 112.

The receive RF section 112 converts the frequency of a radio frequencysignal output from the duplexer 110 to baseband, and outputs theresulting signal to a despreader 113 and a despreader 115.

Despreader 113 despreads the pilot signal component of the basebandsignal and outputs the resulting signal to a CIR measurement section114. The CIR measurement section 114 measures the CIR of the pilotsignal output from despreader 113, and outputs this to the communicationmode selector 101.

Despreader 115 despreads the data component of the baseband signal andoutputs the resulting signal to the adaptive demodulator 116. Theadaptive demodulator 116 demodulates the output signal from despreader115 in accordance with the directions of the communication mode selector101, and outputs the resulting signal to the adaptive decoding section117. The adaptive decoding section 117 decodes the output signal fromthe adaptive demodulator 116 in accordance with the directions of thecommunication mode selector 101, and obtains receive data.

An error detection section 118 performs a CRC on the receive data, andoutputs a signal indicating the CRC result to the retransmission requestsignal creation section 119. That is to say, if the CRC result is thatan error has not been detected in the receive data, the error detectionsection 118 outputs an OK signal indicating that an error has not beendetected to the retransmission request signal creation section 119, andif the CRC result is that an error has been detected in the receivedata, the error detection section 118 outputs an NG signal indicatingthat an error has been detected to the retransmission request signalcreation section 119. The retransmission request signal creation section119 creates an ACK signal if an OK signal is output from the errordetection section 118, or generates a NACK signal if an NG signal isoutput from the error detection section 118, and outputs the respectivesignal to the NACK signal counting section 120 and modulator 106.

The NACK signal counting section 120 counts, for each communicationmode, the number of NACK signals output before an ACK signal is outputfrom the retransmission request signal creation section 119. In otherwords, the NACK signal counting section 120 counts the number of dataretransmissions for each communication mode. A table rewriting section121 compares the number of retransmissions counted by the NACK signalcounting section 120 with a predetermined threshold value for the numberof retransmissions, and rewrites the contents of the communication modetable 102 based on the result of this comparison.

Next, the operation of a communication terminal that has the aboveconfiguration will be described.

A radio signal transmitted from the base station is received by theantenna 111 of the communication terminal, passes through the duplexer110, and is frequency-converted to baseband by the receive RF section112. The baseband signal is despread by despreader 113 and output to theCIR measurement section 114.

In the CIR measurement section 114, the CIR of the pilot signal outputfrom despreader 113 is measured. Then, in the communication modeselector 101, the communication mode table 102 is referred to and acommunication mode is selected based on the CIR measured by the CIRmeasurement section 114.

Here, the contents set in the communication mode table 102 will bedescribed. FIG. 2 is a drawing showing the contents of the communicationmode table provided in a communication terminal according to Embodiment1 of the present invention. For the sake of explanation, it is hereassumed that communication modes are indicated only by the modulationmethod, and that the coding method is the same for all communicationmodes.

As shown in FIG. 2, communication modes are set in the communicationmode table 102 in correspondence to CIRs, so that a communication modeis selected based on the pilot signal CIR measured by the CIRmeasurement section 114. For example, in the case where the CIR measuredby the CIR measurement section 114 is A [dB].CIR<B [dB], a communicationmode for which the modulation method is QPSK is selected by thecommunication mode selector 101, and a DRC signal with a numbercorresponding to the communication mode is created by the DRC signalcreation section 103.

The DRC signal is modulated by modulator 104, spread by spreader 105 andoutput to the multiplexer 108. In this stage, only the DRC signal isoutput from the multiplexer 108.

The DRC signal output from the multiplexer 108 is frequency-converted toradio frequency by the transmit RF section 109, and is transmitted tothe base station as a radio signal from the antenna 111 via the duplexer110.

Next, a radio signal transmitted from the base station in accordancewith the communication mode requested by the communication terminal isreceived by the antenna 111 of the communication terminal, passesthrough the duplexer 110, and is frequency-converted to baseband by thereceive RF section 112. The baseband signal is despread by despreader115, and the data-part signal is output to the adaptive demodulator 116.

In addition, the baseband signal is despread by despreader 113, and thepilot signal is output to the CIR measurement section 114. In the CIRmeasurement section 114 the CIR of the pilot signal is measured, and isoutput to the communication mode selector 101. In the communication modeselector 101 the communication mode is selected as described above.

The data-part signal is demodulated by the adaptive demodulator 116using the demodulation method indicated by the communication modeselector 101, decoded by the adaptive decoding section 117 using thedecoding method indicated by the communication mode selector 101, andoutput to the error detection section 118.

As CRC bits have been added to the data-part signal, a CRC is carriedout on the data-part signal by the error detection section 118. By thismeans it is detected whether or not there is an error in the data-partsignal, and a signal indicating the result of the detection (that is, anOK signal or an NG signal) is output to the retransmission requestsignal creation section 119.

In the retransmission request signal creation section 119, an ACK signalis created if the signal output from the error detection section 118 isan OK signal, or a NACK signal is created if the signal output from theerror detection section 118 is an NG signal, and the created signal isoutput to the NACK signal counting section 120 and modulator 106.

The ACK signal or NACK signal is modulated by modulator 106, spread byspreader 107, multiplexed with the DRC signal by the multiplexer 108,and output to the transmit RF section 109. The output signal from themultiplexer 108 is frequency-converted to radio frequency by thetransmit RF section 109, and is transmitted to the base station as aradio signal from the antenna 111 via the duplexer 110.

In the NACK signal counting section 120, the number of times a NACKsignal is output from the retransmission request signal creation section119 is counted for the currently selected communication mode.

Then, in the base station, if an ACK signal is received the next data istransmitted to the communication terminal, or if a NACK signal isreceived the same data as previously transmitted is retransmitted to thecommunication terminal.

As a result of repeating the above-described operations, the NACK signalcounting section 120 counts, for the currently selected communicationmode, the number of NACK signals output before an ACK signal is outputfrom the retransmission request signal creation section 119. That is tosay, in the NACK signal counting section 120 the number ofretransmissions of data transmitted from the base station issuccessively counted for the currently selected communication mode. Whenan ARQ signal is output from the retransmission request signal creationsection 119, the NACK signal counting section 120 count result is resetto 0.

Then, in the table rewriting section 121, the number of retransmissionscounted by the NACK signal counting section 120 is compared with apredetermined threshold value N, and the contents of the communicationmode table 102 are rewritten based on the result of this comparison.Here, the operation of the table rewriting section 121 will bedescribed. FIG. 3 is an operational flowchart for explaining theoperation of the table rewriting section provided in a communicationterminal according to Embodiment 1 of the present invention.

In the table rewriting section 121, the number of retransmissionscounted by the NACK signal counting section 120 is compared with apredetermined threshold value N in Step (hereinafter abbreviated to“ST”) 201. Here, the threshold value N is the maximum number ofretransmissions permitted in the system, and this permissible value N ispredetermined based on the desired reception quality of data-partsignals required in the system.

If, in ST201, the number of retransmissions is less than N, thedata-part signal reception quality can be said to be excessive qualitythat exceeds the desired reception quality required in the system. Thatis to say, the current actual downlink channel quality can be consideredto have improved since the point at which channel quality was measuredby the CIR measurement section 114. It can therefore be determined that,with the current actual downlink channel quality, communication can beperformed using a communication mode with a higher transmission ratethan the communication mode selected based on the pilot signal CIR.

Thus, if the number of retransmissions is less than N in ST201, thecontents of the communication mode table 102 shown in. FIG. 2 arerewritten as shown in ST202 by the table rewriting section 121. That isto say, A [dB], B [dB], and C [dB] set in the communication mode table102 shown in FIG. 2 are decremented respectively by predetermined valuesX [dB], Y [dB], and Z [dB]. As a result, the communication mode selector101 selects a communication mode with a higher transmission rate thanthe previously selected communication mode, even if the pilot signal CIRmeasured by the CIR measurement section 114 is the same.

If, on the other hand, the number of retransmissions is greater than Nin ST201, the data-part signal reception quality can be said not toattain the desired reception quality required in the system. That is tosay, the current actual downlink channel quality can be considered tohave degraded since the point at which channel quality was measured bythe CIR measurement section 114. It can therefore be determined that,with the current actual downlink channel quality, it is necessary forcommunication to be performed using a communication mode with a lowertransmission rate than the communication mode selected based on thepilot signal CIR.

Thus, if the number of retransmissions is greater than N in ST201, thecontents of the communication mode table 102 shown in FIG. 2 arerewritten as shown in ST204 by the table rewriting section 121. That isto say, A [dB], B [dB], and C [dB] set in the communication mode table102 shown in FIG. 2 are incremented respectively by predetermined valuesX [dB], Y [dB], and Z [dB]. As a result, the communication mode selector101 selects a communication mode with a lower transmission rate than thepreviously selected communication mode, even if the pilot signal CIRmeasured by the CIR measurement section 114 is the same.

If the number of retransmissions is equal to N in ST201, the data-partsignal reception quality can be said to be on a level with the desiredreception quality required in the system, and therefore the contents ofthe communication mode table are not rewritten, as shown in ST203.

Actual examples of the communication mode table rewrite operationsdescribed above are illustrated in FIG. 4A through FIG. 4C. FIG. 4Athrough FIG. 4C are drawings showing examples of communication modetable rewrite operations by the table rewriting section of acommunication terminal according to Embodiment 1 of the presentinvention. Explanations are given here for the case where A [dB], B[dB], and C [dB] currently set in the communication mode table 102 shownin FIG. 2 are 4 [dB], 8 [dB], and 12 [dB], respectively, and variationamounts X [dB], Y [dB], and Z [dB] are all 1 [dB].

First, in FIG. 4A, A [dB], B [dB], and C [dB] are set to 4 [dB], 8 [dB],and 12 [dB], respectively. Then, if the number of retransmissions isless than N, A [dB], B [dB], and C [dB] are all decremented by 1 [dB]each, and the communication mode table 102 is rewritten as shown in FIG.4B. If, on the other hand, the number of retransmissions is greater thanN, A [dB], B [dB], and C [dB] are all incremented by 1 [dB] each, andthe communication mode table 102 is rewritten as shown in FIG. 4C.

In this way, based on the number of retransmissions of a data-partsignal (that is, the data-part signal reception quality), the tablerewriting section 121 detects that a difference has arisen between thechannel quality measured by the CIR measurement section 114 and thecurrent actual channel quality, and rewrites the contents of thecommunication mode table 102.

Thus, according to this embodiment, the reception quality of a data-partsignal is determined by the number of times a NACK signal is sent back,and the contents of a communication mode table indicating thecorrespondence between downlink channel quality and communication modesare rewritten based on the number of times a NACK signal is sent back,thereby making it possible to select a communication mode that enablescommunication to be performed most efficiently with the current actualchannel quality.

Also, according to this embodiment, data-part signal reception qualityis determined according to the number of retransmissions based on a CRC,so that reception quality determination can be performed quickly andeasily, enabling communication mode table rewrites to be performed athigh-speed, keeping up with variations in channel quality.

Moreover, according to this embodiment, communication mode tablerewrites are performed with reference to a maximum number ofretransmissions permitted in the system. In other words, communicationmode table rewrites are performed with reference to a desired receptionquality required in the system. Thus, according to this embodiment, itis possible to perform downlink data communication while maintaining thedesired reception quality required in the system.

In this embodiment, it is also possible for the NACK signal countingsection 120 to calculate an average value of the number ofretransmissions at predetermined intervals for each communication mode,and for the table rewriting section 121 to rewrite the communicationmode table 102 based on the result of a comparison between. that averagevalue of the number of retransmissions and a predetermined thresholdvalue N. The reliability of the number of retransmissions is improved bycalculating an average value of the number of retransmissions in thisway, enabling communication mode table rewrites to be performedaccurately and without errors.

(Embodiment 2)

A communication terminal according to Embodiment 2 of the presentinvention determines the reception quality of a data-part signal bymeans of the error rate, and based on this error rate, rewrites thecontents of a communication mode table indicating the correspondencebetween downlink channel quality and communication modes.

FIG. 5 is a block diagram showing the configuration of a communicationterminal according to Embodiment 2 of the present invention. As shown inthis figure, a communication terminal according to this embodimentdiffers from the communication terminal shown in FIG. 1 in beingprovided with an error rate calculation section 301 and table rewritingsection 302 instead of an error detection section 118, retransmissionrequest signal creation section 119, NACK signal counting section 120,and table rewriting section 121. In the following description, partsidentical to those in FIG. 1 are assigned the same codes as in FIG. 1and their detailed explanations are omitted.

In FIG. 5, a communication mode selector 101 refers to a communicationmode table 102 and selects a communication mode based on a CIR measuredby a CIR measurement section 114, and outputs this to a DRC signalcreation section 103 and the error rate calculation section 301.

The error rate calculation section 301 calculates the error rate of thedata-part signal output from an adaptive decoding section 117 for eachcommunication mode, and outputs this to the table rewriting section 302.Here, the Bit Error Rate (BER) or BLock Error Rate (BLER) is used as theerror rate calculated by the error rate calculation section 301. The biterror rate can be calculated by comparing data-part signals before andafter error correction to detect a bit in which an error has occurred,and the block error rate can be calculated by performing a CRC to detecta block in which an error has occurred. The bit error rate has theadvantage of more accurately indicating the reception quality of adata-part signal than the block error rate, while the block error ratehas the advantage of being able to be calculated with a simplerequipment configuration than the bit error rate.

The table rewriting section 302 compares the error rate calculated bythe error rate calculation section 301 with a predetermined error ratethreshold value, and rewrites the contents of the communication modetable 102 based on the result of the comparison. Here, the predeterminedthreshold value is the error rate permitted in the system, thispermissible value being determined beforehand based on the desireddata-part signal reception quality required in the system.

If the error rate calculated by the error rate calculation section 301is lower than the predetermined threshold value, the data-part signalreception quality can be said to be excessive quality that exceeds thedesired reception quality required in the system. That is to say, thecurrent actual downlink channel quality can be considered to haveimproved since the point at which channel quality was measured by theCIR measurement section 114. It can therefore be determined that, withthe current actual downlink channel quality, communication can beperformed using a communication mode with a higher transmission ratethan the communication mode selected based on the pilot signal CIR.

Thus, if the error rate calculated by the error rate calculation section301 is lower than the predetermined threshold value, A [dB], B [dB], andC [dB] set in the communication mode table 102 shown in FIG. 2 aredecremented by the table rewriting section 302 by predetermined values X[dB], Y [dB], and Z [dB], respectively, in the same way as inabove-described Embodiment 1.

If, on the other hand, the error rate calculated by the error ratecalculation section 301 is higher than the predetermined thresholdvalue, the data-part signal reception quality can be said not to attainthe desired reception quality required in the system. That is to say,the current actual downlink channel quality can be considered to havedegraded since the point at which channel quality was measured by theCIR measurement section 114. It can therefore be determined that, withthe current actual downlink channel quality, it is necessary forcommunication to be performed using a communication mode with a lowertransmission rate than the communication mode selected based on thepilot signal CIR.

Thus, if the error rate calculated by the error rate calculation section301 is higher than the predetermined threshold value, A [dB], B [dB],and C [dB] set in the communication mode table 102 shown in FIG. 2 areincremented by the table rewriting section 302 by predetermined values X[dB], Y [dB], and Z [dB], respectively, in the same way as inabove-described Embodiment 1.

In this way, based on the error rate of a data-part signal (that is, thedata-part signal reception quality), the table rewriting section 302detects that a difference has arisen between the channel qualitymeasured by the CIR measurement section 114 and the current actualchannel quality, and rewrites the contents of the communication modetable 102.

Thus, according to this embodiment, the reception quality of a data-partsignal is determined by the error rate, and the contents of acommunication mode table indicating the correspondence between downlinkchannel quality and communication modes are rewritten based on thiserror rate, thereby making it possible to select a communication modethat enables communication to be performed most efficiently with thecurrent actual channel quality.

Also, according to this embodiment, determining data-part signalreception quality by means of the error rate enables data-part signalreception quality to be determined more accurately. Therefore,communication mode table rewrites can be performed accurately andwithout errors.

(Embodiment 3)

A communication terminal according to Embodiment 3 of the presentinvention determines the reception quality of a data-part signal bymeans of data-part signal throughput, and based on this throughput,rewrites the contents of a communication mode table indicating thecorrespondence between downlink channel quality and communication modes.

FIG. 6 is a block diagram showing the configuration of a communicationterminal according to Embodiment 3 of the present invention. As shown inthis figure, a communication terminal according to this embodimentdiffers from the communication terminal shown in FIG. 1 in beingprovided with a throughput calculation section 401 and table rewritingsection 402 instead of an error detection section 118, retransmissionrequest signal creation section 119, NACK signal counting section 120,and table rewriting section 121. In the following description, partsidentical to those in FIG. 1 are assigned the same codes as in FIG. 1and their detailed explanations are omitted.

In FIG. 6, a communication mode selector 101 refers to a communicationmode table 102 and selects a communication mode based on a CIR measuredby a CIR measurement section 114, and outputs this to a DRC signalcreation section 103, the throughput calculation section 401, and thetable rewriting section 402.

The throughput calculation section 401 calculates the average throughputof the data-part signal output from an adaptive decoding section 117 atpredetermined intervals for each communication mode, and outputs this tothe table rewriting section 402. As [Mbps] is normally used as the unitof throughput, the throughput calculation section 401 can calculate theaverage data-part signal throughput by finding the average number ofdata-part signal bits received per second.

The table rewriting section 402 compares the average throughputcalculated by the throughput calculation section 401 with apredetermined throughput threshold value, and rewrites the contents ofthe communication mode table 102 based on the result of the comparison.One method of calculating the predetermined throughput value isdescribed below.

If the number of terminals currently simultaneously communicating with abase station (hereinafter referred to as “number of simultaneouslycommunicating terminals”) is N, of the signals transmitted from the basestation, on average 1/N can be assumed to be signals transmitted to theterminal being considered. Therefore, if a communication mode isselected that is expected to enable a throughput of 2 [Mbps] to beattained, for example, an average throughput of 1.2/N [Mbps] can beexpected to be attained by a communication terminal for which that modeis selected. This 1.2/N [Mbps] throughput is then the above-mentionedpredetermined threshold value.

Thus, the table rewriting section 402 calculates a predeterminedthreshold value for each communication mode based on the communicationmode output from the communication mode selector 101 and the number ofsimultaneously communicating terminals, and compares the averagethroughput calculated for each communication mode by the throughputcalculation section 401 with the corresponding predetermined thresholdvalue. It is assumed that terminals are notified of the number ofsimultaneously communicating terminals by the base station.

The predetermined throughput threshold value is not limited to thatdescribed above, but may, for example, be determined beforehand on thebasis of desired data-part signal reception quality required in thesystem, as in above-described Embodiments 1 and 2.

If the average throughput calculated by the throughput calculationsection 401 is higher than the predetermined threshold value, thecurrent actual downlink channel quality can be considered to haveimproved since the point at which channel quality was measured by theCIR measurement section 114. It can therefore be determined that, withthe current actual downlink channel quality, communication can beperformed using a communication mode with a higher transmission ratethan the communication mode selected based on the pilot signal CIR.

Thus, if the average throughput calculated by the throughput calculationsection 401 is higher than the predetermined threshold value, A [dB], B[dB], and C [dB] set in the communication mode table 102 shown in FIG. 2are decremented by the table rewriting section 402 by predeterminedvalues X [dB], Y [dB], and Z [dB], respectively, in the same way as inabove-described Embodiment 1.

If, on the other hand, the average throughput calculated by thethroughput calculation section 401 is lower than the predeterminedthreshold value, the current actual downlink channel quality can beconsidered to have degraded since the point at which channel quality wasmeasured by the CIR measurement section 114. It can therefore bedetermined that, with the current actual downlink channel quality, it isnecessary for communication to be performed using a communication modewith a lower transmission rate than the communication mode selectedbased on the pilot signal CIR.

Thus, if the average throughput calculated by the throughput calculationsection 401 is lower than the predetermined threshold value, A [dB], B[dB], and C [dB] set in the communication mode table 102 shown in FIG. 2are incremented by the table rewriting section 402 by predeterminedvalues X [dB], Y [dB], and Z [dB], respectively, in the same way as inabove-described Embodiment 1.

In this way, based on the average throughput of a data-part signal (thatis, the data-part signal reception quality), the table rewriting section402 detects that a difference has arisen between the channel qualitymeasured by the CIR measurement section 114 and the current actualchannel quality, and rewrites the contents of the communication modetable 102.

Thus, according to this embodiment, the reception quality of a data-partsignal is determined by the data-part signal throughput, and thecontents of a communication mode table indicating the correspondencebetween downlink channel quality and communication modes are rewrittenbased on this throughput, thereby making it possible to select acommunication mode that enables communication to be performed mostefficiently with the current actual channel quality.

Also, throughput is a value that indicates actual reception quality in acommunication terminal more accurately than the number ofretransmissions or the error rate. Therefore, rewriting thecommunication mode table based on throughput enables communication modetable rewrites to be performed more accurately.

(Embodiment 4)

In above-described Embodiments 1 to 3, a communication terminal selectsa communication mode based on the pilot signal CIR, and transmits a DRCsignal corresponding to that selected communication mode to the basestation. DRC signal information can be represented with far fewer bitsthan other information indicating downlink channel quality (such as adownlink CIR, for example), and therefore use of a DRC signal has theadvantage of enabling the downlink channel utilization ratio to beincreased. On the other hand, since a communication terminal has toselect the communication mode and create a DRC signal, and must beprovided with a table for communication mode selection, a table for DRCsignal creation, and so forth, there are the disadvantages of increasedcommunication terminal power consumption and equipment scale.

Thus, in the embodiment described below, a communication terminaltransmits a CIR signal indicating the pilot signal CIR to the basestation, and the base station refers to a communication mode table andselects a communication mode based on the CIR. As a result, althoughthere is the disadvantage of a slight decrease in the uplink channelutilization ratio, the fact that communication terminals do not have toselect the communication mode and create a DRC signal, and do not needto be provided with a communication mode selection table, DRC signalcreation table, and so forth, offers the major advantage of enablingcommunication terminal power consumption and equipment scale to bereduced. Also, in the embodiment described below, it is possible forCIRs transmitted from a plurality of terminals to be compared in thebase station, and the correct communication mode to be determined withcertainty, making the embodiment described below particularly useful incases such as those where it is hot possible for the communication modeto be determined simply from the CIR in each communication terminal.

This embodiment will be described below. A base station according toEmbodiment 4 rewrites the contents of a communication mode tableindicating the correspondence between downlink channel quality andcommunication modes based on the number of times a NACK signal is sentback from a communication terminal.

FIG. 7 is a block diagram showing the configuration of a communicationterminal that performs radio communication with a base station accordingto Embodiment 4 of the present invention. In the following description,parts identical to those in FIG. 1 are assigned the same codes as inFIG. 1 and their detailed explanations are omitted.

In FIG. 7, a CIR signal creation section 501 creates a CIR signal thatindicates the pilot signal CIR measured by a CIR measurement section114, and outputs this signal to a modulator 104. Modulator 104 modulatesthe CIR signal and outputs it to a spreader 105.

A despreader 502 despreads a baseband signal with the spreading codeused to despread a signal indicating the communication mode, and outputsthe despread signal to a communication mode detection section 503. Thecommunication mode detection section 503 demodulates the output signalfrom the despreader 502 and detects the communication mode. Then, basedon the detected communication mode, the communication mode detectionsection 503 indicates the outbound receive data demodulation method toan adaptive demodulator 116 and indicates the outbound receive datadecoding method to an adaptive decoding section 117.

FIG. 8 is a block diagram showing the configuration of a base stationaccording to Embodiment 4 of the present invention.

In FIG. 8, an allocation section 601 determines communication resourceallocation to each communication terminal based on a CIR indicated by aCIR signal extracted by a demodulator 616 described later herein. Then,based on the determined communication resource allocation, theallocation section 601 gives an instruction to a buffer 606 for outputof outbound transmit data, and outputs the CIR signal to a communicationmode selector 602.

The communication mode selector 602 refers to a communication mode table603, selects a communication mode based on the CIR indicated by the CIRsignal output from the allocation section 601, and outputs a signalindicating that communication mode to a modulator 604 and NACK signalcounting section 619. Also, based on the selected communication mode,the communication mode selector 602 indicates the outbound transmit datacoding method to an adaptive coding section 607 and indicates theoutbound transmit data modulation method to an adaptive modulator 608.The contents set in the communication mode table 603 are identical tothose shown in FIG. 2, and therefore a description thereof will beomitted here. A communication mode table 603 is provided for eachcommunication terminal.

The modulator 604 modulates the signal indicating the communicationmode, and outputs it to a spreader 605. Spreader 605 spreads the outputsignal from the modulator 604 and outputs the resulting signal to amultiplexer 610. The buffer 606 holds outbound transmit data, andoutputs outbound transmit data for a predetermined communicationterminal to the adaptive coding section 607 in accordance withdirections from the allocation section 601. The adaptive coding section607 codes the output signal from the buffer 606 in accordance withdirections from the communication mode selector 602, and outputs theresulting signal to the adaptive modulator 608. The adaptive modulator608 modulates the output signal from the adaptive coding section 607 inaccordance with directions from the communication mode selector 602, andoutputs the resulting signal to a spreader 609. Spreader 609 spreads theoutput signal from the adaptive modulator 608, and outputs the resultingsignal to the multiplexer 610. The multiplexer 610 multiplexes thesignal indicating the communication mode with outbound transmit data,and outputs the resulting signal to a receive RF section 611. Thereceive RF section 611 converts the frequency of the output signal fromthe multiplexer 610 to radio frequency and outputs it to a duplexer 612.

The duplexer 612 transmits the output signal from the transmit RFsection 611 to the communication terminals as a radio signal via anantenna 613. In addition, the duplexer 612 outputs a signal transmittedas a radio signal from a communication terminal and received as a radiosignal by the antenna 613 to a receive RF section 614. The receive RFsection 614 converts the frequency of a radio frequency signal outputfrom the duplexer 612 to baseband, and outputs the resulting signal to adespreader 615 and a despreader 617. Despreader 615 despreads thebaseband signal with the spreading code used to spread the CIR signal,and outputs the resulting signal to a demodulator 616. Demodulator 616demodulates the output signal from despreader 615 and extracts the CIRsignal, which it outputs to the allocation section 601.

Despreader 617 despreads the baseband signal with the spreading codeused to spread an ACK signal or NACK signal, and outputs the resultingsignal to a demodulator 618. Demodulator 618 demodulates the outputsignal from despreader 617 and extracts an ACK signal or NACK signal,which it outputs to the NACK signal counting section 619. The NACKsignal counting section 619 counts, for each communication mode, thenumber of NACK signals output before an ACK signal is output fromdemodulator 618. In other words, the NACK signal counting section 619counts the number of data retransmissions for each communication mode.

A despreader 615, demodulator 616, despreader 617, demodulator 618, andNACK signal counting section 619 are provided for each communicationterminal. A CIR signal for each communication terminal is output fromthe corresponding demodulator 616, and the number of dataretransmissions is counted by the respective NACK signal countingsections 619 for each communication terminal and for each communicationmode.

A table rewriting section 620 compares the number of retransmissionscounted by the NACK signal counting section 619 with a predeterminedthreshold value for the number of retransmissions, and rewrites thecontents of the communication mode table 603 for the relevantcommunication terminal based on the result of this comparison.

Next, the operation of a base station with the above configuration willbe described.

An ACK signal or NACK signal transmitted from a communication terminalis demodulated by demodulator 618 and output to the NACK signal countingsection 619. In the NACK signal counting section 619, the number of NACKsignals output before an ACK signal is output from demodulator 618 iscounted for the currently selected communication mode. That is to say,in the NACK signal counting section 619 the number of retransmissions ofdata to the communication terminal is successively counted for thecurrently selected communication mode. When an ARQ signal is output fromdemodulator 618, the NACK signal counting section 619 count result isreset to 0.

Then, in the table rewriting section 620, the number of retransmissionscounted by the NACK signal counting section 619 is compared with apredetermined threshold value N, and the contents of the communicationmode table 603 for the relevant communication terminal are rewrittenbased on the result of this comparison. The table rewriting section 603rewrite operation is as described in Embodiment 1 above, and so adescription of this operation will be omitted here.

Thus, according to this embodiment, in the same way as inabove-described Embodiment 1, the contents of a communication mode tableindicating the correspondence between downlink channel quality andcommunication modes are rewritten based on the number of times a NACKsignal is sent back from a communication terminal, thereby enabling thesame kind of effect to be obtained as with above-described Embodiment 1.

(Embodiment 5)

A communication terminal according to Embodiment 5 of the presentinvention rewrites the contents of a communication mode table indicatingthe correspondence between downlink channel quality and communicationmodes based on the data-part signal error rate notified by acommunication terminal.

A base station according to this embodiment will be described below.FIG. 9 is a block diagram showing the configuration of a communicationterminal that performs radio communication with a base station accordingto Embodiment 5 of the present invention. In the following description,parts identical to those in FIG. 7 are assigned the same codes as inFIG. 7 and their detailed explanations are omitted.

In FIG. 9, an error rate detection section 701 detects the error rate ofa data-part signal output from an adaptive decoding section 117, andoutputs this to a notification signal creation section 702. The detailedoperation of the error rate detection section 701 is as described inEmbodiment 2 above, and so a description of this operation will beomitted here.

The notification signal creation section 702 creates a signal indicatingthe error rate and outputs this signal to a modulator 106. The signalindicating the error rate is modulated by modulator 106, spread by aspreader 107, multiplexed with a CIR signal by a multiplexer 108, andtransmitted to the base station.

FIG. 10 is a block diagram showing the configuration of a base stationaccording to Embodiment 5 of the present invention. In the followingdescription, parts identical to those in FIG. 8 are assigned the samecodes as in FIG. 8 and their detailed explanations are omitted.

In FIG. 10, a communication mode selector 602 outputs a signalindicating the selected communication mode to a modulator 604 and errorrate detection section 802. A despreader 801 despreads a baseband signalwith the spreading code used to spread the signal indicating the errorrate, and outputs the resulting signal to the error rate detectionsection 802. The error rate detection section 802 demodulates the outputsignal from the despreader 801 and extracts the signal indicating theerror rate, and detects the error rate of the data-part signal in eachcommunication terminal for each communication mode.

A despreader 615, demodulator 616, despreader 801, and error ratedetection section 802 are provided for each communication terminal. ACIR signal for each communication terminal is output from thecorresponding demodulator 616, and the data-part signal error rate isdetected by the respective error rate detection section 802 for eachcommunication terminal and for each communication mode.

A table rewriting section 803 compares the error rate detected by theerror rate detection section 802 with a predetermined error ratethreshold value, and rewrites the contents of the communication modetable 603 for the relevant communication terminal based on the result ofthis comparison.

Next, the operation of a base station with the above configuration willbe described.

A signal indicating the error rate transmitted from a communicationterminal is demodulated by the error rate detection section 802. By thismeans the data-part signal error rate is detected. The detected errorrate is output to the table rewriting section 803.

Then, in the table rewriting section 803, the error rate detected by theerror rate detection section 802 is compared with a predeterminedthreshold value, and the contents of the communication mode table 603for the relevant communication terminal are rewritten based on theresult of this comparison. The table rewriting section 603 rewriteoperation by the table rewriting section 803 is as described inEmbodiment 2 above, and so a description of this operation will beomitted here.

Thus, according to this embodiment, in the same way as inabove-described Embodiment 2, the contents of a communication mode tableindicating the correspondence between downlink channel quality andcommunication modes are rewritten based on the data-part signal errorrate notified by a communication terminal, thereby enabling the samekind of effect to be obtained as with above-described Embodiment 2.

(Embodiment 6)

A communication terminal according to Embodiment 6 of the presentinvention rewrites the contents of a communication mode table indicatingthe correspondence between downlink channel quality and communicationmodes based on data-part signal throughput notified by a communicationterminal.

A base station according to this embodiment will be described below.FIG. 11 is a block diagram showing the configuration of a communicationterminal that performs radio communication with a base station accordingto Embodiment 6 of the present invention. In the following description,parts identical to those in FIG. 7 are assigned the same codes as inFIG. 7 and their detailed explanations are omitted.

In FIG. 11, a throughput calculation section 901 calculates the averagethroughput of the data-part signal output from an adaptive decodingsection 117 at predetermined intervals, and outputs this to anotification signal creation section 902. The method of calculating theaverage throughput is as described in Embodiment 3 above, and so adescription of this method will be omitted here.

The throughput calculation section 901 creates a signal indicating theaverage throughput and outputs this signal to a modulator 106. Thesignal indicating the average throughput is modulated by modulator 106,spread by a spreader 107, multiplexed with a CIR signal by a multiplexer108, and transmitted to the base station.

FIG. 12 is a block diagram showing the configuration of a base stationaccording to Embodiment 6 of the present invention. In the followingdescription, parts identical to those in FIG. 8 are assigned the samecodes as in FIG. 8 and their detailed explanations are omitted.

In FIG. 12, a communication mode selector 602 outputs a signalindicating the selected communication mode to a modulator 604 andthroughput detection section 1002. A despreader 1001 despreads abaseband signal with the spreading code used to spread the signalindicating the average throughput, and outputs the resulting signal tothe throughput detection section 1002. The throughput detection section1002 demodulates the output signal from the despreader 1001 and extractsthe signal indicating the average throughput, and detects the averagethroughput of the data-part signal in each communication terminal foreach communication mode.

A despreader 615, demodulator 616, despreader 1001, and throughputdetection section 1002 are provided for each communication terminal. ACIR signal for each communication terminal is output from thecorresponding demodulator 616, and the data-part signal averagethroughput is detected by the respective throughput detection section1002 for each communication terminal and for each communication mode.

A table rewriting section 1003 compares the average throughput detectedby the throughput detection section 1002 with a predetermined throughputthreshold value, and rewrites the contents of the communication modetable 603 for the relevant communication terminal based on the result ofthis comparison.

Next, the operation of a base station with the above configuration willbe described.

A signal indicating the average throughput transmitted from acommunication terminal is demodulated by the throughput detectionsection 1002. By this means the data-part signal average throughput isdetected. The detected average throughput is output to the tablerewriting section 1003.

Then, in the table rewriting section 1003, the average throughputdetected by the throughput detection section 1002 is compared with apredetermined threshold value, and the contents of the communicationmode table 603 for the relevant communication terminal are rewrittenbased on the result of this comparison. The table rewriting section 603rewrite operation by the table rewriting section 1003 is as described inEmbodiment 3 above, and so a description of this operation will beomitted here.

Thus, according to this embodiment, in the same way as inabove-described Embodiment 3, the contents of a communication mode tableindicating the correspondence between downlink channel quality andcommunication modes are rewritten based on data-part signal throughputnotified by a communication terminal, thereby enabling the same kind ofeffect to be obtained as with above-described Embodiment 3.

In above-described Embodiments 1 to 6, the CIR of a pilot signal is usedas a value indicating downlink channel quality, but this is not alimitation, and any value may be used as long as it is a value thatindicates channel quality.

Also, in above Embodiments 1 to 6, in order to prevent the communicationmode table from being rewritten frequently, a threshold value given apredetermined width may be set as the threshold value to be comparedwith the number of retransmissions, error rate, or average throughput.For example, it is possible to set two new threshold valuesincremented/decremented by ±X with respect to the threshold value usedin above Embodiments 1 to 6, and to arrange for rewriting of thecommunication mode table not be performed if the number ofretransmissions, error rate, or average throughput is within a range of±X with respect to the threshold value used in above Embodiments 1 to 6.

Moreover, in above Embodiments 1 to 6, a threshold value to be comparedwith the number of retransmissions, error rate, or average throughputmay be set for each communication mode.

Furthermore, in above Embodiments 1 to 3, a communication terminal maybe notified by the base station of the threshold value to be comparedwith the number of retransmissions, error rate, or average throughput.

In addition, in above Embodiments 1 to 6, all the CIR values set in thecommunication mode table are rewritten when the communication mode tableis rewritten, but a particular CIR value or plurality of CIR values maybe rewritten instead.

Also, in above Embodiments 1 to 6, the variation widths of the CIRvalues set in the communication mode table are assumed to be fixedvalues (X [dB], Y [dB], and Z [dB]), but it is also possible forvariation widths to be varied adaptively according to the size of thedifference between the measured channel quality and the current actualchannel quality.

Moreover, in above Embodiments 1 to 6, the fact that a difference hasarisen between the measured channel quality and the current actualchannel quality is detected on the basis of data-part signal channelquality, but this is not a limitation, and any method may be used aslong as it is a method that can detect the fact that a difference hasarisen.

Thus, according to above Embodiments 1 to 6, in a communication systemin which communication resources are allocated to communicationterminals by time division based on downlink channel quality measuredfrom a pilot signal, the correspondence between downlink channel qualityand communication modes is rewritten when a difference arises betweenthe measured channel quality and the current actual channel quality,thereby making it possible to select a communication mode that enablescommunication to be performed most efficiently with the current actualchannel quality. Thus, according to the present invention, it ispossible to prevent a fall in downlink throughput.

(Embodiment 7)

FIG. 13 is a block diagram showing the configuration of a base stationaccording to Embodiment 7 of the present invention, and FIG. 14 is ablock diagram showing the configuration of a communication terminalaccording to Embodiment 7 of the present invention.

First, in a modulator/spreader 1204 of the base station shown in FIG.13, a pilot burst signal is modulated and then spread. This spread pilotburst signal is multiplexed with other signals by a multiplexer 1203,and the resulting signal undergoes predetermined transmission processingsuch as up-conversion by an RF section 1202, and is then transmittedfrom an antenna 1201.

This signal is received by the antenna 1301 of the communicationterminal shown in FIG. 14, undergoes predetermined reception processingsuch as down-conversion by an RF section 1302, and is then output to adespreader/demodulator 1303. In the despreader/demodulator 1303, thereceived signal is despread and then demodulated, and is output to a CRCsection 1304 and CIR measurement section 1306.

In the CIR measurement section 1306, the reception quality to bespecific, the CIR of the pilot burst signal in the demodulated receivedsignal is measured. The measured CIR is output to a rate request valuedetermination section 1307. The correspondence between CIRs andtransmission rates has been stored beforehand in the rate request valuedetermination section 1307, and the transmission rate corresponding tothe measured CIR is selected by the CIR measurement section 1306. Thisselected transmission rate is then output to a multiplexer 1309 as therate request value of this terminal.

In the multiplexer 1309, the rate request value and transmit data fromthis terminal are multiplexed, and this multiplex signal is modulatedand then spread by a modulator/spreader 1310. This spread signalundergoes predetermined transmission processing by the RF section 1302,and is then transmitted from the antenna 1301.

This signal is received by the antenna 1201 of the base station shown inFIG. 13, undergoes predetermined reception processing by the RF section1202, and is then output to a despreader/demodulator 1208. In thedespreader/demodulator 1208, the received signal is despread and thendemodulated, and is output to a TPC signal generation section 1211 andradio resource management section 1212.

In the TPC signal generation section 1211, a TPC signal for controllingterminal transmission power is generated using a pilot symbol includedin the demodulated signal. This TPC signal is assembled into a MACchannel signal by a MAC channel assembly section 1210. The MAC channelsignal is modulated and then spread by a modulator/spreader 1206, and isoutput to the multiplexer 1203.

In the radio resource management section 1212, the communicationterminal that transmitted the largest rate request value among the raterequest values from all the communication terminals is selected, and theresult of this selection is output to a buffer section 1216, dedicatedchannel coding section 1209, and modulator/spreader 1205. The selectionmethod may also be to select the communication terminal that requestedthe lowest transmission rate, so that communication is possible for allcommunication terminals. There are no particular restrictions on theselection method.

Transmit data for this selected communication terminal is read in thebuffer section 1216. Then, in the dedicated channel coding section 1209,address information indicating which communication terminal thistransmit data is destined for is added to this read transmit data. Thenthe data with address information added is modulated and then spread bymodulator/spreader 1205, and is output to the multiplexer 1203. Each ofthe signals output to multiplexer 1203 is multiplexed and thentransmitted from the RF section 1202 via the antenna 1201.

When this signal is received by the communication terminal shown in FIG.14, if address information for this terminal is received, the signalfollowing the address information is received. Then a CRC is performedon the receive data by the CRC section 1304. If the result of this CRCis OK, the receive data is output via a decomposition section 1305 to alatter-stage circuit (not shown). If the result of this CRC is NG, onthe other hand, the receive data is not output to the decompositionsection 1305. The CRC result (OK or NG) is transmitted to the basestation shown in FIG. 13 via the multiplexer 1309, modulator/spreader1310, RF section 1302, and antenna 1301.

When this CRC result (OK or NG) is received by the base station shown inFIG. 13, it is input to a downlink error measurement section 1213. Inthe downlink error measurement section 1213, the error rate of signaltransmission to the communication terminal is estimated from this CRCresult, and this error rate is output to a rate change request section1214.

Estimation of the error rate is performed as follows. The number of NGswithin a predetermined interval is counted for each communicationterminal and for each allocated data rate. Similarly, the number oftimes allocated is counted. Then the result of dividing the number ofNGs by the number of times allocated is taken as the error rateestimate.

In the rate change request section 1214, the error rate is compared witha predetermined first threshold value and a predetermined secondthreshold value. It is assumed that the second threshold value is alower value than the first threshold value. It is also assumed that thesecond threshold value is greater than zero and almost zero.

Then, in the rate change request section 1214, if the error rate isoutside the predetermined range—that is, greater than or equal to thefirst threshold value or less than or equal to the second thresholdvalue—it is determined that the transmission rate determined from theCIR by the rate request value determination section 1307 is incorrect.That is to say, it is determined that the rate request value from thecommunication terminal is incorrect.

In other words, in the rate change request section 1214, if the errorrate is greater than or equal to the first threshold value, it isdetermined that the rate request value is too high and the desiredcommunication quality cannot be attained at that transmission rate. Inthis case, therefore, a signal instructing the communication terminal tolower the rate request value below the transmission rate determined fromthe CIR is generated by the rate change request section 1214 as a ratechange instruction signal. In this way the base station can cause thetransmission rate requested by the communication terminal to be changedto a transmission rate that meets the desired communication quality.

If, on the other hand, the error rate is less than or equal to thesecond threshold value, it is determined that the rate request valuefrom the communication terminal is too low and communication quality isexcessive at that transmission rate. In this case, therefore, a signalinstructing the communication terminal to raise the rate request valueabove the transmission rate determined from the CIR is generated by therate change request section 1214 as a rate change instruction signal. Inthis way the base station can cause the transmission rate requested bythe communication terminal to be changed to a transmission rate at whichdata communication can be performed more efficiently.

Determination and rate change instruction operations performed by therate change request section 1214 may be divided between two componentsections (a determination section and a changing section).

A rate change instruction signal generated by the rate change requestsection 1214 is assembled into a control channel signal by a controlchannel assembly section 1215. The control channel signal is modulatedand then spread by a modulator/spreader 1207, and transmitted from theRF section 1202 via the multiplexer 1203.

When this control channel signal is received by the communicationterminal shown in FIG. 14, the signal undergoes a CRC by the CRC section1304, and if the result is OK, is output to the decomposition section1305. In the decomposition section 1305, the control channel signal isdecomposed and the rate change instruction signal extracted, and theextracted rate change instruction signal is output to a rate requestvalue changing section 1308.

A rate request value change instruction is issued by the rate requestvalue changing section 1308 to the rate request value determinationsection 1307 in accordance with the rate change instruction signal. Inthe rate request value determination section 1307, the rate requestvalue is changed in accordance with this instruction. That is to say, ifthe instruction given by the rate change instruction signal is aninstruction to lower the rate request value, the rate request value islowered below the transmission rate determined from the CIR, and,conversely, if the instruction given by the rate change instructionsignal is an instruction to raise the rate request value, the raterequest value is raised above the transmission rate determined from theCIR. In this way it is possible for a transmission rate request valuedetermined by a communication terminal to be changed by thatcommunication terminal in accordance with an instruction from the basestation.

Thus, according to this embodiment, if the error rate is outside apredetermined range, a base station determines that the transmissionrate requested by a communication terminal is incorrect and instructsthe communication terminal to change the transmission rate requestvalue, and the communication terminal changes the transmission raterequest value in accordance with that instruction. By this means it ispossible for the transmission rate used for data transmission from thebase station to a communication terminal to be made a transmission ratethat enables the desired communication quality to be attained. In otherwords, the transmission rate value can be made a value that enables datatransmission to be performed appropriately. Thus, appropriate datatransmission can be performed, and communication failures can beeliminated.

As described above, according to the present invention it is possible toprevent a fall in downlink throughput in a communication system in whichcommunication resources are allocated to communication terminals by timedivision based on downlink channel quality measured from a pilot signal.

This application is based on Japanese Patent Application No. 2000-232269filed on Jun. 26, 2000, and Japanese Patent Application No. 2000-249554filed on Aug. 21, 2000, entire content of which is expresslyincorporated by reference herein.

1. A base station apparatus comprising: an estimator that estimates anerror rate of reception data in a communication terminal apparatus; atransmission rate changer that, when the estimated error rate is greaterthan or equal to an upper limit value of a predetermined range,generates a signal to instruct the communication terminal apparatus tolower a transmission rate; and a transmitter that transmits thegenerated signal to the communication terminal apparatus.
 2. The basestation apparatus according to claim 1, wherein the estimator estimatesthe error rate using CRC results of the reception data in thecommunication terminal apparatus.
 3. The base station apparatusaccording to claim 2, wherein the estimator finds a value of theestimated error rate by dividing the number of NGs in the CRC results bythe number of times the transmission rate is allocated.
 4. A basestation apparatus comprising: an estimator that estimates an error rateof reception data in a communication terminal apparatus; a transmissionrate changer that, when the estimated error rate is less than or equalto a lower limit value of a predetermined range, generates a signal toinstruct the communication terminal apparatus to raise a transmissionrate; and a transmitter that transmits the generated signal to thecommunication terminal apparatus.
 5. The base station apparatusaccording to claim 4, wherein the estimator estimates the error rateusing CRC results of the reception data in the communication terminalapparatus.
 6. The base station apparatus according to claim 5, whereinthe estimator finds a value of the estimated error rate by dividing thenumber of NGs in the CRC results by the number of times the transmissionrate is allocated.
 7. A radio communication method comprising:estimating an error rate of reception data in a communication terminalapparatus; generating a signal to instruct the communication terminalapparatus to lower a transmission rate, when the estimated error rate isgreater than or equal to an upper limit value of a predetermined range;and transmitting the generated signal to the communication terminalapparatus.
 8. A radio communication method comprising: estimating anerror rate of reception data in a communication terminal apparatus;generating a signal to instruct the communication terminal apparatus toraise a transmission rate, when the estimated error rate is less than orequal to a lower limit value of a predetermined range; and transmittingthe generated signal to the communication terminal apparatus.